The current invention relates to optical spectrometers, especially interferometric optical spectrometers. More particularly, the invention is a method and a device to measure the modified or unmodified spectrum of a light source with high resolution. Basically, there are two ways to get access to information about the spectral content of an electromagnetic wave in the optical range and the adjacent parts of the frequency spectrum, resulting in two kinds of optical spectrometer in existence. One option is to spectrally disperse the incoming radiation by a dispersive element like a prism or a grating. The so obtained frequency-dependent intensity pattern can then be imaged onto a detector with spatial resolution (e.g. a CCD or CMOS camera). If an appropriate relay imaging is involved, the complete spectrum within the bandwidth of the apparatus can be observed simultaneously. Usually, these spectrometers have an entrance slit; and the resolving power of the device increases with decreasing slit width. Consequently, the performance of such a device is a compromise between resolving power and detection threshold for low light intensity. A common embodiment of such a device is a Czerny-Turner spectrograph, comprising a rotatable plane grating between mirrors or lenses that image the entrance slit to the exit slit.
The second option is to use interference of the electric field of the radiation under test. For this, either a reference wave with known spectrum has to be used, or the radiation to be tested is split in two or more parts and an autocorrelation of the wave with itself is performed. The second way is usually preferred, since a reference source would severely limit the bandwidth or the operation range of the device. Commonly, the corresponding interferometric spectrometers are set up as Michelson or Mach-Zehnder interferometers. One of the two beams is temporally delayed with respect to the other and a variation of this delay yields a time-dependent interference pattern, which can be converted to a spectrum by way of Fourier transform. A common disadvantage of time-delay based Fourier-Transform Spectrometers (as opposed to dispersive spectrometers) is that all spectral components contribute noise to the measured signal. Fourier Transform Spectrometers and Fabry-Perot spectrometers are common embodiments of the interferometric kind, the latter one being an example for an input spectrum that is compared to a fixed reference spectrum (which is the transmission spectrum of the Fabry-Perot setup).
A further version of interferometric spectrometers is realized by generating Fizeau fringes, the spatial frequency of which contains information about the spectrum of the radiation under test. Here too, at least two beams are generated from the incoming light and are brought to interference under a defined angle. From the interferogram, the spectrum may again be obtained by way of a numerical Fourier transform. Differently from the Fourier transform spectrometers that use a temporal delay between the two interfering spectral functions, these spectrometers can collect the complete spectrum within the bandwidth of the device simultaneously without the need to move any parts.
A modern version of these Fizeau interferometers is the spatial heterodyne spectrometer (U.S. Pat. Nos. 5,059,027; 7,119,905; 7,330,267; 7,466,421; 7,773,229; and US Patent Applications with Publication Nos. 20050046858; 20090231592; 20100321688; 20130188181; 20140029004; 20150030503), where a reflective diffraction grating under Littrow angle causes the necessary wavefront tilt for close off-Littrow wavelengths. These spectrometers can be built compact and without moving parts, however, they generally require optical elements of high quality. They can realize a large resolving power due to the fact that fringe spectrum is heterodyned around the Littrow wavelength.
Another version, a modified Sagnac spectrometer (U.S. Pat. Nos. 7,433,044 and 8,736,844) is similar to the spatial heterodyne spectrometer in that it is built with at least two reflection gratings. The two interfering beams are the two counter-propagating beams of the Sagnac ring interferometer; the resulting fringe spectrum is heterodyned around the Littrow wavelength as well. In this device as well, high-quality optical components are necessary. If the wavelength range of this spectrometer needs to be changed on a continuous basis, high-precision rotational stages for the gratings are required, due to the sensitive dependence of the Littrow wavelength on the angle of incidence on the grating.